In EMFC weighing cells the weight of the load is transmitted either directly or by way of one or more force-transmitting levers to an electromechanical measurement transducer which generates an electrical signal corresponding to the weight of the load, which is processed and sent to a display by an electronic weighing signal processor.
Weighing cells with a measurement transducer based on elastic deformation contain a deformable body to which strain gauges are attached. The deformable body changes its shape elastically under the applied load. In many cases, the deformable body is configured as a parallelogram transducer, i.e. a parallel-motion linkage with specially configured bending zones whereby defined zones of deformation, specifically bending zones, are set up where the strain gauges are arranged. As the strain gauges are stretched or compressed as a result of the deformation, they exhibit a change in their respective electrical resistance values in comparison to the stress-free state, and the resistance change represents a measure for the applied load.
In weighing cells with oscillating strings, the mechanical design concept is largely analogous to the EMFC weighing cells, with the difference that an oscillating-string measurement transducer is used instead of an electromagnetic transducer. By applying the weighing load, an oscillating string is put under tension and the resultant change of its resonance frequency represents again a measure for the applied load.
One characteristic trait of the weighing cells of the foregoing description, which all balances with a parallel-guided weighing pan (as opposed to a freely suspended weighing pan) have in common, is the property that the weight force transmitted from the weighing pan to the transducer generally shows a slight dependency on whether the weighing load is placed on the center of the weighing pan or off-centered towards the edge. This can have the undesirable consequence that a balance indicates a different amount of weight for one and the same weighing load, depending on where the load was placed on the weighing pan. These deviations which occur when the weighing load is placed eccentrically on the weighing pan are commonly called eccentric load errors.
In a parallelogram transducer or in a parallel-guiding mechanism which guides the weighing pan support in a parallel movement by means of two guide members which are parallel to each other and essentially horizontal, eccentric load errors occur primarily due to the fact that the parallel-guiding members deviate slightly from the ideal, absolutely parallel alignment. The relative magnitude of the eccentric load error, i.e. the ratio between the observed weighing error and the size of the test weight being used, approximately corresponds to the relative geometrical deviation which caused the eccentric load error. A distinction is made between an eccentric load error in the lengthwise direction and an eccentric load error in the transverse direction of the parallel-guiding mechanism, in accordance with the direction in which the test weight is moved on the weighing pan in an eccentric load test of the balance. An eccentric load error in the lengthwise direction occurs if the vertical distance between the parallel-guiding members at the end which is connected to the stationary parallelogram leg is not exactly equal to the distance at the opposite end which is connected to the movable parallelogram leg. An eccentric load error in the transverse direction, on the other hand, occurs if the two parallel-guiding members are slightly twisted relative to each other, i.e. if the distance between the parallel-guiding members varies over the width of the parallel-guiding members.
It is known that in weighing cells of the type disclosed for example in U.S. Pat. No. 6,232,567 B1, the parallelism deviations of the parallel guides and, consequently, the eccentric load errors associated with them can be corrected by removing material from the bending zones of the parallel guides by grinding or filing. A removal of material from the topside causes the effective center of rotation of the flexure pivot to be offset in the downward direction, while a removal of material from the underside of the bending zone will offset its effective center of rotation in the upward direction.
A further solution for the correction of eccentric load errors can be found, e.g., in WO 2005/031286, wherein by means of an adjusting device formed in the monolithic material block, the end of the upper parallel-guiding member which is connected to the stationary parallel leg can be raised or lowered as well as adjusted in regard to its transverse angle through adjustment screws, whereby the eccentric load errors in the lengthwise as well as the transverse direction of the weighing cell can be corrected.
A way to adjust eccentric load errors is also disclosed in JP 2002365125 A, wherein at two locations an adjustment zone with an adjustment flexure fulcrum is arranged between a flexure pivot and a parallel leg, and wherein each adjustment zone is connected to the first lever end of an adjustment lever and the adjustment flexure fulcrum serves at the same time as the fulcrum of the adjustment lever.
The adjustment of eccentric load errors by removing material from the flexure pivots presents a problem in weighing cells which are designed for precision balances and analytical balances, i.e. for small weighing loads and high resolutions, and which therefore have slender flexure pivots. The grinding or filing to remove material from a thin flexure pivot requires a sensitive touch. This operation is therefore in most cases performed manually and is accordingly cost-intensive.
Likewise, total freedom from problems is not achieved with the adjustment of eccentric load errors in the foregoing examples of WO 2005/031286 and JP 2002365125 A, wherein an adjustment device is formed on the stationary parallelogram leg by cuts and bored holes in the monolithic material block, and wherein by the tightening or loosening of adjustment screws the adjustment device can be deformed in such a way that the end of the upper parallel-guiding member that is connected to the adjustment device can be raised or lowered as well as twist-adjusted in its transverse angle. Even with the slightest corrective change, the adjustment device as disclosed in these references will cause considerable stresses in the material of the parallel-guiding mechanism. Over a longer time period, an age-related change can occur in the eccentric load adjustment as a result of stress relaxation in the material sectors which have been elastically biased in one direction or the other by means of the adjustment screws. Reversible short-term changes can be caused by temperature fluctuations, if the monolithic material block and the adjustment screws have different coefficients of thermal expansion.
A way to adjust eccentric load errors which is likewise based on elastic deformation is disclosed in DE 202 17 590 U1. By means of a screw exerting a pull on a lever-like projection, a bending moment is applied to a part which holds on one side a parallel-guiding member through a flexure pivot and carries on the opposite side the lever-like projection. Although the reduction ratio between the vertical movements of the free end of the projection and of the pivot location of the parallel-guiding member can be adapted to requirements through an appropriate choice of dimensions, it should be kept in mind that the part to which the bending moment is applied has to withstand the horizontal forces of the parallel-guiding member. It remains an essential drawback of this eccentric load adjustment that the adjustment range is very limited. Accordingly, the parallel-guiding mechanism needs to be manufactured with a commensurate level of precision so that the limited adjustment range will be sufficient to correct the remaining errors.
In commonly-owned U.S. Pat. No. 7,851,713, to one of the co-inventors here (Burkhard), a solution to these unsatisfactory aspects of the eccentric load adjustment in parallel-guiding mechanisms is proposed through the concept of providing an adjustment zone with an adjustment flexure fulcrum which is subjected to a controlled plastic deformation for the adjustment of parallelism. This eccentric load adjustment has proved itself exceptionally well in practice; the gravimetric measuring instruments equipped with it no longer have a temperature-dependent eccentric load error. Even those drift phenomena that manifest themselves over longer time periods could be reduced significantly, so that the intervals between calibrations could be extended.
The accuracy of the adjustment that can be achieved by plastically deforming the adjustment flexure fulcra is sufficient for gravimetric measuring instruments with a measurement resolution as fine as one milligram. In measuring instruments with a finer resolution, however, an adjustment of sufficient accuracy cannot be achieved because of changes in material properties that occur as a result of cold-forming.
A desired objective is to further develop the concept of the plastically deformable adjustment zone taught by Burkhard '713, the aim being to open the way to a more precise adjustment of the parallel-guiding mechanism while keeping the positive characteristics of the plastically deformable adjustment zone.